Re: [PATCH V3 1/3] vfio, platform: add support for ACPI while detecting the reset driver

From: okaya
Date: Tue Mar 29 2016 - 08:16:04 EST

On 2016-03-29 07:25, Arnd Bergmann wrote:
On Tuesday 29 March 2016 06:59:15 okaya@xxxxxxxxxxxxxx wrote:
On 2016-03-29 05:25, Arnd Bergmann wrote:
> On Monday 28 March 2016 09:35:22 Sinan Kaya wrote:
>> The code is using the compatible DT string to associate a reset driver
>> with
>> the actual device itself. The compatible string does not exist on ACPI
>> based systems. HID is the unique identifier for a device driver
>> instead.
>> The change allows a driver to register with DT compatible string or
>> HID and then match the object with one of these conditions.
>> Rules for loading the reset driver are as follow:
>> - ACPI HID needs match for ACPI systems
>> - DT compat needs to match for OF systems
>> Tested-by: Eric Auger <eric.auger@xxxxxxxxxx> (device tree only)
>> Tested-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> (ACPI only)
>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> This really feels wrong for two reasons:
> * device assignment of non-PCI devices is really special and doesn't
> seem to make sense on general purpose servers that would be the
> target
> for ACPI normally

Why is it special? Acpi is not equal to pci. Platform devices are first
class devices too. Especially, _cls was introduced for this reason.

It still feels like a hack. The normal design for a server is to have
all internal devices show up on the PCI host bridge, next to the PCIe
ports, to have a simple way to manage any device, both internal and
off-chip. Putting a device on random MMIO registers outside of the
discoverable buses and have the firmware work around the lack of
discoverability will always be inferior.

It is a HW implementation choice. Having everything as pci problem has been already solved. I would vote for it when we had SW pci bridge layer just to use usb and sata. Not anymore. Especially, _cls solves this problem

> * If there is indeed a requirement for ACPI to handle something like
> this,
> it should be part of the ACPI spec, with a well-defined method of
> handling
> reset, rather than having to add a device specific hack for each
> device separately.

I see. Normally, this is done by calling _rst method. AFAIK, Linux
doesnât support _rst. I can check its presence and call it if it is

Yes, that sounds reasonable: In patch 2 where you check for the
presence of the reset method, just keep the existing logic for
DT based systems, and use _rst on ACPI based systems instead,
then you can drop both patches 1 and 3.

I can certainly drop patch #3 and push the reset responsibility to acpi.

I never liked having a fragmented sw design across multiple drivers.

I need something for patch #1. Compatible is a DT property not ACPI.but then, I won't have a reset driver anymore.

If we think about how vfio pci works, we pass the pci vendor and device id to new_id file to find out which pci device needs to be pass thru.

I can go to a similar route. This time we pass the object id through new_id and I call reset method on this object.

Let me know what you think?